Internal combustion engines can produce exhaust streams that include various gases and combustion products. Some of these gases, such as nitrogen oxide gases (NOx) including, for example, nitrogen monoxide (NO) and nitrogen dioxide (NO2), can contribute to environmental pollution in the form of acid rain and other undesirable effects. As a result, many regulations have been imposed on engine manufacturers in an attempt to reduce the levels of NOx emitted into the atmosphere.
NOx removal from the exhaust streams of lean burn engines can be especially challenging. Lean burn engines, which may include diesel engines as well as certain spark ignited engines, can operate with an excess of oxygen. Specifically, in a lean burn engine, more oxygen may be supplied to the engine than is necessary to stoichiometrically consume the fuel admitted to the engine. As a result, the exhaust streams of these lean burn engines may be rich in oxygen, which can limit the available techniques suitable for NOx removal.
To reduce the NOx concentrations in the exhaust stream of lean burning engines, a number of lean-NOx catalysts have been developed that may selectively reduce NOx in oxygen rich exhaust streams with hydrocarbon reductants. These lean-NOx catalytic systems may depend on the presence of sufficient levels of hydrocarbon species to be fully effective. The amount of hydrocarbons available in the exhaust streams of many lean burning engines can be low. Therefore, in some applications including active catalytic systems, a hydrocarbon compound such as diesel fuel, for example, may be introduced into the exhaust stream in order to promote reduction of NOx compounds.
Several lean-NOx catalysts have been developed that include alumina in some form. Alumina is known as a durable material, and it has shown promise as a catalyst for lean-NOx reactions at high temperatures. Nevertheless, even alumina-based catalysts have proven problematic. For example, certain catalysts or catalytic systems that have been used with lean burn engines can suffer from low NOx conversion efficiencies, inadequate catalyst durability, low thermal stability, narrow effective temperature ranges, and NOx selectivity limited to only certain compounds. Further, these catalysts and catalytic systems may be subject to sulfur poisoning from even minimal amounts of sulfur present in some fuels and certain lubricants. For example, sulfur, in the form of SO2 present in an exhaust stream, can significantly reduce the NOx conversion effectiveness of a lean-NOx catalyst or catalytic system.
In an attempt to address the shortcomings of lean-NOx catalysts, various catalyst configurations and compositions have been proposed. For example, U.S. Pat. No. 5,980,844 (“the '844 patent”) describes a NOx-reducing catalyst that includes silver oxide particles dispersed on alumina. The combination of the silver oxide particles and the alumina is meant to address the tendency of lean-NOx catalysts to deactivate in the presence of SO2 when used to reduce NOx in automotive exhaust gases.
While the '844 patent addresses one deficiency of traditional lean-NOx catalysts, it fails to take into account the effects of various NOx gases and supplemental reductants in the exhaust stream. Further, the production of the small, widely dispersed silver oxide particles requires complex processing that can add to the manufacturing costs of the catalyst.